1. Field of the Invention
The present invention relates to touch sensing transducers and systems. More particularly, the present invention relates to flexible and transparent object position recognition devices useful in applications such as cursor movement and user input for computing devices and other applications.
2. Description of the Related Field
Touch sensing technology can be used to provide a variety of input to electronic devices, from emulating physical buttons or slider switches to more sophisticated recognition of patterns or gestures. For example, U.S. Pat. No. 5,880,411 to Gillespie, which is hereby incorporated in its entirety for its teaching of touch sensor technology, discloses a capacitive sensor trace array object position detector with edge motion feature and gesture recognition. U.S. Pat. No. 5,305,017 to Gerpheide teaches an object position detector using capacitive sensing and is also incorporated in its entirety by reference for its teaching of touch sensor technology. Another example of capacitive touchpads is U.S. Pat. No. 5,543,588 to Bisset.
However, in many applications it is desirable to display the surface underlying the touchpad. For example, the touchpad can be overlaid on an active display such as a LCD or CRT screen to facilitate input to a graphical user interface (GUI). Alternatively, it may be desired to have the underlying surface display static information such as a button pattern to guide user input or advertising. Neither Gillespie nor Bisset disclose or suggest the ability to display a surface underlying the touchpad sensor.
The general strategy employed by the prior art to provide transparent touch sensing technology for portable devices utilizes a resistive pad. A typical resistive pad consists of two substantially transparent substrates separated by a thin air gap, typically about 0.006 inches. One substrate is deformable, such as a polyester film, while the other is typically rigid, such as glass or polycarbonate plastic. Both substrates are coated with a substantially transparent, conductive coating like Indium Tin Oxide on the surface where they face one another. On the ITO-coated surfaces, one substrate contains conductors at its left and right edges while the other substrate contains conductors at its top and bottom. To measure position in one axis, a voltage gradient is applied to one set of conductors while voltage is measured on the other set. When an object deforms the resistive pad so that one substrate contacts the other, the point of contact will contain a voltage proportional to the distance from the conductor where the voltage was applied. To measure position in the other axis, the conductors for voltage application and measurement are switched.
There are other variations of the previously described 4 wire resistive pad sensors. One example is a 5-wire design which alternately applies an X-axis, than Y-axis voltage gradient to the one layer and then uses the second layer simply to sense voltage.
Different prior art techniques deal with the difficulty of maintaining a constant air gap between the two ITO layers and the stretching of the deformable substrate as it wears. For example, substantially transparent dielectric spacer dots, usually about 0.010″ in diameter, can be arranged in a regular, grid-like fashion over one of the ITO surfaces. These dots help maintain the air gap to prevent the two ITO layers from accidentally contacting. Further, the dots help counteract the decreasing performance of the deformable substrate as it wears and stretches. However, the use of these spacer dots generally reduces the resolution of the touchpad, as not all locations of such a resistive pad will necessarily generate a signal when the two substrates come in contact.
In addition to the resistive pad technologies, other examples of prior art include capacitance based technologies, stylus based technologies and pressure actuation technologies. Prior art capacitive touchpads such as U.S. Pat. No. 5,457,289 to Huang and U.S. Pat. Nos. 4,806,709 and 4,733,222 to Evans suffer from various drawbacks of their own. For example, Huang requires frontal shielding for its capacitive touch sensor system and the Evans '709 requires a uniformly electrically conductive surface. Stylus based pads such as U.S. Pat. No. 5,381,160 to Landmeir and U.S. Pat. No. 4,945,348 to Ibamoto require an electromagnetic tool to provide input. Finally, pressure actuated touchpads such as U.S. Pat. No. 4,484,038 to Dorman and U.S. Pat. No. 3,757,322 to Barkan have moving parts and thus suffer from drawbacks similar to the resistive pad technologies.